The objective of this Mentored Research Scientist Development Award (K01) is to provide Dr. Angela Robertson Bazzi with the training, mentoring, research experience, and protected time needed to support her transition to research independence in the development of prevention interventions for marginalized populations affected by substance use. Training goals (TGs) include developing skills and expertise in intervention development (TG1), implementation science (TG2), research ethics (TG3), and professional development for a successful academic research career (TG4). Training will be accomplished through an intensive combination of formal structured courses, training workshops and institutes, directed readings, applied research experience, and interactions with experienced mentors who are renowned experts in intervention development, implementation science, substance use, and antiretroviral pre-exposure prophylaxis (PrEP) for HIV prevention. The overarching objective of the candidate?s research is to develop a community- based PrEP intervention for people who inject drugs (PWID). Although PrEP is recommended for HIV prevention among PWID, uptake in this high risk and socially marginalized population has been low, and acceptability and adherence challenges remain poorly understood. This revised proposal, which is guided by a model of healthcare utilization for vulnerable populations, a systematic approach to intervention development, and implementation science frameworks, will meet the following specific aims: Aim 1: identify the modifiable determinants of PrEP access and utilization among HIV-uninfected PWID (n=30) and key informants (n=15); Aim 2: develop a manualized intervention (TG1) to improve PrEP uptake and adherence among PWID attending a community-based syringe exchange program (SEP); and Aim 3: conduct a pilot randomized controlled trial (RCT) of the resulting SEP-based PrEP uptake and adherence intervention for PWID (n=50) with a mixed methods process evaluation focused on implementation outcomes (TG2). Despite the existence of effective HIV prevention strategies for PWID (e.g., access to sterile syringes through SEPs), the increasing prevalence of opioid misuse over the past decade is threatening past achievements in HIV prevention. The high proportion of U.S. PWID who report past-year receptive syringe sharing and/or condomless intercourse (77%) highlights a need for expanded delivery of PrEP, which is efficacious and recommended for HIV prevention among PWID. By situating this K01 on the Boston University Medical Campus, which is co-located with community-based health and social service organizations, the candidate will be able to access a large population of PWID and leading experts in intervention development and implementation science. While the proposed pilot trial may not be powered to detect changes in PrEP uptake or adherence outcomes, it will provide the necessary skills and preliminary data to determine intervention feasibility, maximize stakeholder acceptability, and infer effect size estimates and implementation aspects for a future full-scale R01 study.